A shaft position sensor as disclosed herein is typically used with a digitally controlled stepper motor which drives a hydraulic or pneumatic control valve, such as a butterfly valve, for controlling airflow in an aircraft cabin environmental control system. The sensor may also be used with a butterfly valve in an engine fuel control system, as the case may be.
The disclosed invention verifies whether or not the motor shaft has taken a step in response to a command input and provides information which may be fed back to a digital controller or the like to determine discrepancies between actual and commanded stepper motor shaft displacement. In this regard it will be appreciated that a single digital controller can monitor a multitude of such information for presentation on a single cathode ray tube (CRT) display to the pilot of the aircraft, and thereby provides a continuous verification of the motor shaft position. By way of comparison, analog systems for the purposes described typically generate a valve position feedback signal for display on a dedicated cockpit gauge.
Heretofore digital systems for sensing stepper motor shaft position have used optical decoders or the like mounted to the shaft of the stepper motor. This arrangement, with its additional hardware, increases the cost of the sensor, while decreasing its reliability.
The present invention senses displacement of a stepper motor shaft by sensing a particular signature of the motor current, i.e. the derivative of the current when the motor is excited. If the derivative of the current does not fall below a certain level during a certain time the motor shaft has not taken a step.
Prior art arrangements relating to sensors used in conjunction with motors of which the present inventor is aware are described in U.S. Pat. Nos. 2,654,860; 3,267,346; 3,384,796; 4,422,040; 4,305,028; and 4,275,342. The inventor is also aware of U.K. Patent Specification No. 429,478 and Japanese Patent No. 59-35598.
U.S. Pat. No. 2,654,860 issued to Lewis in 1953 senses a change in load for an AC motor as a function of motor current. The present invention senses movement of a stepper motor shaft using sensed motor current and hence is distinguished from the sensor of the patent.
U.S. Pat. No. 3,267,346 issued to Liang, et al in 1966 uses an external sensor, i.e. a carrier means and a switch, to sense shaft position in order to synchronize pulses applied to a sensor motor and is thus distinguished from the present invention.
U.S. Pat. No. 3,384,796 issued to Shah in 1968 relates to a control arrangement for stopping a drive motor and uses a tachometer for sensing the rate of change of speed of the motor, and accordingly is also distinguished from the present invention.
U.S. Pat. No. 4,422,040 issued to Raider, et al in 1983 relates to testing stepper motors, wherein the back EMF of the motor is compared to desired motor characteristics, and is hence not seen as directed to the present invention.
U.S. Pat. No. 4,305,028 issued to Kostas, et al in 1981 relates to an arrangement for evaluating whether or not a robot can perform a series of programmed motions by comparing the velocity of the links of a robot simulator against predetermined velocity limits which will reproduce programmed steps. This patent is likewise not seen as directed to the present invention.
U.S. Pat. No. 4,275,342 issued to Kwada, et al in 1981 relates to control apparatus for a DC motor wherein a tachometer output is sensed with motor voltage. If one occurs without the other an alarm is generated. This patent is also not seen as directed to the present invention.
U.K. Patent Specification No. 429,478 dated May, 1985, and in the name of CHF Muller Aktiengesellschaft relates to sensing movement of a rotor subjected to the stator field of an induction motor and is hence seen to be different than the present invention.
Japanese Pat. No. 59-35598 dated February, 1984 and in the name of Fujitsu, et al relates to sensing the stepping of a pulse motor. The patent teaches determining whether or not the motor shaft has taken a step by looking at the motor current to see if the current reaches a particular level within a particular time. If this occurs, the motor shaft has not taken a step. The present invention is distinguished over the arrangement of the patent in that whether or not the motor has taken a step is determined by looking at the derivative of the motor current. This arrangement has distinct advantages over merely looking at the current as in the Japanese patent. For example, extraneous influences can easily affect the current waveform as observed in the Japanese patent, and thereby give a false or ambiguous indication as to whether or not the motor shaft has taken a step. This ambiguity is eliminated when the derivative of the current waveform is observed as in the present invention, and hence a more accurate and reliable sensor is provided. Accordingly, the present invention is seen to distinguish over the teachings of the Japanese patent.